<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
            width: 100%;
            height: 100%;
        }
        #btn-1 {
            width: 15px;
            height: 20px;
            background: red;
        }

        #btn-2 {
            width: 15px;
            height: 20px;
            background: green;
        }

        #btn-3 {
            width: 15px;
            height: 20px;
            background: aquamarine;
        }
    </style>

</head>
<body>
<button id="btn-1" onclick="btnone()"></button>
<button id="btn-2" onclick="btntwo()"></button>
<button id="btn-3" onclick=" btnthree()"></button>
<script>
    var btn1 = document.getElementById("btn-1")
    var btn2 = document.getElementById("btn-2")
    var btn3 = document.getElementById("btn-3")
    var bodys = document.getElementsByTagName("body")[0]
    console.log(bodys)

    function btnone() {
        bodys.style.background = 'red'
        btn1.style.border = "5px solid #000"
        btn2.style.border = "0px"
        btn3.style.border = "0px"
    }
    function btntwo() {
        bodys.style.background = 'green'
        btn2.style.border = "5px solid #000"
        btn1.style.border = "0px"
        btn3.style.border = "0px"
    }
    function btnthree() {
        bodys.style.background = 'aquamarine'
        btn3.style.border = "5px solid #000"
        btn1.style.border = "0px"
        btn2.style.border = "0px"
    }
</script>
</body>
</html>